1. Field of the Invention
The present invention relates to a toner for developing electrostatically charged images in an electrophotographic method, an electrostatic-printing recording method, and the like. More particularly, it relates to a conductive magnetic toner for use in a conductive developing method.
2. Prior Art
In general, an electrophotographic method comprises the steps of: (1) forming an electric latent image on a photoconductor; (2) developing the latent image with toners to form a toner image; (3) optionally transferring the toner image to a paper; and (4) fixing the toner image by means of heating, pressurization, and the like to obtain a copy. Classes of developers for use in such electrophotographic methods include two-component developers consisting of a toner and a carrier, and single-component developers consisting of only a toner which also functions as a carrier.
The single-component developers have two main classes. One is a magnetic single-component developer, and the other is a non-magnetic single-component developer. The magnetic single-component developer includes magnetic powder in the amount of approximately from 10% by weight to 70% by weight. In addition, the magnetic toners are roughly divided into conductive magnetic toners and insulating magnetic toners. The driving force of the conductive magnetic toner is an electrostatic induction or an electric charge. On the other hand, the driving force of the insulating magnetic toner is an electric charge by means of triboelectrification.
It has been known that the magnetic single-component development using the conductive magnetic toner has advantages in that there is no need to control the toner density and that the uniform image density without edge effect can be obtained. In addition, if the conductive magnetic toner has the specific resistance of generally not more than 10.sup.4 .OMEGA..cm, the magnetic toner can be advantageously used in a system having an electric potential of not more than 200 V.
On the other hand, the conductive magnetic toner has disadvantages in that the toner is liable to leak the charges thereof via the transfer paper during the electrostatic transfer, for which reason, it is difficult to transfer the image to the plain paper. In addition, if large amounts of carbon blacks are added to the magnetic toner so as to provide conductive properties to the magnetic toner, it is disadvantageously difficult for the toner to be thermally fixed.
Although in order to dissolve the transfer problem described above, the special papers wherein the high-resistance treatment has been carried out, or the pressure transfer method can be employed, the thermal fixing properties are not adequate. Therefore, in the conventional art, both low resistance and good fixing properties cannot be obtained simultaneously.
In order to obtain both the desired low-resistance and fixing properties, it has been proposed that large amounts of carbon black are added to the surfaces of the high-resistant toner particles having the specific resistance of about from 10.sup.6 .OMEGA..cm to 10.sup.9 .OMEGA..cm, or that the conductive carbon blacks are fixed to the surfaces of the same toner particles as described above by means of impulse force. When the proposals described above are applied to the conductive toners, the seemingly specific resistance is lowered. However, in fact, the internal resistance of the magnetic toner is not so lowered. For this reason, conductive parts are not sufficiently formed in the developing step and the electric charges are not adequately poured. In particular, in the low developing-potential system, there are disadvantages such as reduction of the image density, increase in the fog density, and the like.